System and Method for Forming Nano-Particles in Additively-Manufactured Metal Alloys

ABSTRACT

In some embodiments, a method of producing a metallic article includes providing a metallic powder, selecting a predetermined concentration for a reactive component, providing a controlled atmosphere including the reactive component at the predetermined concentration, and additively manufacturing the metallic article from the metallic powder under the controlled atmosphere. The metallic powder includes a metallic element or metallic alloy. The reactive component reacts with the metallic powder in a weld pool formed during the additive manufacturing to form a dispersion of nano-particles in the weld pool. The nano-particles are dispersed throughout the metallic article in a substantially uniform manner. In some embodiments, the metallic powder includes the reactive component. Metallic articles formed by the disclosed methods are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is an application under 35 USC 111(a) and claimspriority under 35 USC 119 from Provisional Application Ser. No.62/491,792, filed Apr. 28, 2017 under 35 USC 111(b). The disclosure ofthat provisional application is incorporated herein by reference.

FIELD OF THE INVENTION

The described invention relates in general to additive manufacturingprocesses, and more specifically to a system and method for formingnano-particles in certain types of metal alloys, particularly thosecreated by beam melting processes such as selective laser melting andelectron beam melting.

BACKGROUND OF THE INVENTION

When a cast pure metal or alloy is permanently deformed in any manner,it is considered a wrought metal. Wrought metals are mostly base metalalloys, such as stainless steel, cobalt-chromium-nickel,nickel-titanium, and beta-titanium. Because of plastic deformation, themicrostructure of an alloy is altered and the alloy exhibits mechanicalproperties that are different from those it had in the as-cast state.The most significant changes are its proportional limit and ductility.Ductility refers to a solid material stretching under tensile stress. Ifductile, a material may be stretched into a wire or similar structure.Malleability, a similar mechanical property, is a material's ability todeform under pressure (compressive stress). If malleable, a material maybe flattened by hammering or rolling.

Precipitation hardening, also called age hardening, is a heat treatmenttechnique used to increase the yield strength of malleable materials,including most structural alloys of aluminum, magnesium, nickel,titanium, and some steels and stainless steels. The process ofprecipitation hardening produces uniformly dispersed particles (i.e.,precipitates) within the grain structure of a metal that hinderdislocation motion, thereby strengthening the metal and enhancing itsmechanical properties. These precipitates are generally conventionallyformed using a two-step process: (i) an initial solution heat treatment(i.e. solutionization) at a high temperature; and (ii) a precipitationhardening treatment at a lower temperature, which results in theformation of precipitates. The solution heat treatment results in asingle-phase solution, and the precipitation hardening treatment resultsin the formation of precipitates of appropriate size and morphology toenhance certain mechanical properties of the material. Precipitationhardening is typically performed in a vacuum having an inert atmosphereat temperatures ranging from 900° F. to 1150° F. (480° C. to 620° C.)for steel alloys to below 400° F. (200° C.) for some aluminum alloys.The process ranges in time from one to twenty-four hours depending onthe specific material and specified characteristics.Precipitation-hardened alloys may be used for a variety of applications,including those where prolonged exposure to elevated temperatures orother harsh environments may occur.

As an alternative to using wrought alloys for creating components orparts, selective laser melting (SLM) and electron beam melting (EBM) areadditive manufacturing techniques that utilize a directed energy toselectively fuse together a predetermined portion of the surface of abed of powdered metal material. SLM uses a laser beam as the directedenergy, whereas EBM uses an electron beam as the directed energy. Thedirected energy source is automatically aimed at points in space thatare defined by a series of horizontal layers within a three-dimensional(3D) model, thereby melting/welding the powdered metal into a solidstructure. SLM and EBM machines are commercially available and utilize a3D computer-aided design (CAD) model, where a data file is created andthen sent to the machine's processing unit. A technician typicallymanipulates this 3D model to best orient the geometry for part buildingand adds one or more support structures as appropriate. Once this “buildfile” has been completed, it is “sliced” into the horizontal layers thatthe machine builds in and is downloaded to the machine, allowing thebuild to commence.

Both selective laser melting and electron beam melting operate inside anenvironmentally-controlled build chamber that also includes a materialdispensing system, a build platform, and a re-coater blade that is usedto move new powder over the build platform. The environment inside thechamber is conventionally either an inert gas or a vacuum. Metal powderis fused into a solid part by melting or welding the powder locallyusing the directed energy beam. Parts are built up additively layer bylayer, typically using layers between about 20 to 100 micrometers (0.8to 3.9 mil) thick. Such beam melting processes permit highly complexpart geometries to be created directly from 3D CAD data, in a fullyautomated manner, in a relatively short period of time, and without anytooling.

Selective laser melting and electron beam melting are net-shapeprocesses that produces parts with high accuracy and detail resolution,good surface quality, and excellent mechanical properties. These beammelting processes have many benefits over traditional manufacturingtechniques, including the ability to rapidly produce a unique partwithout any special tooling being required. They also provide for morerigorous testing of prototypes due to the fact that they are useablewith most alloys. Accordingly, functional prototypes can be created fromthe same material as production components. Such systems may also beused in full-scale production in addition to prototyping. Becausecomponents are built additively, i.e., layer by layer, it is possible toinclude internal features and passages that could not be cast orotherwise machined. Beam melting processes are used to manufacturedirect parts for a variety of industries including aerospace, dental,medical, and other industries that required small to medium size, highlycomplex parts.

Components or parts made using the beam melting processes may also beprecipitation hardened to increase strength and durability. However,components or parts made by these processes may possess certainadvantageous characteristics compared to the wrought forms of the sameor similar alloys. Accordingly, there is an ongoing need forunderstanding and enhancing the systems and methods used in suchprocesses to enable the creation of materials having superiorperformance characteristics.

BRIEF DESCRIPTION OF THE INVENTION

The following provides a summary of certain exemplary embodiments of thepresent invention. This summary is not an extensive overview and is notintended to identify key or critical aspects or elements of the presentinvention or to delineate its scope.

In accordance with one aspect of the present invention, a method ofproducing a metallic article includes providing a metallic powder,selecting a predetermined concentration for a reactive component,providing a controlled atmosphere including the reactive component atthe predetermined concentration, and additively manufacturing themetallic article from the metallic powder under the controlledatmosphere. The metallic powder includes a metallic element or metallicalloy. The reactive component reacts with the metallic powder in a weldpool formed during the additive manufacturing to form a dispersion ofnano-particles in the weld pool. The nano-particles are dispersedthroughout the metallic article in a substantially uniform manner.

In accordance with another aspect of the present invention, a method ofproducing a metallic article includes selecting a predeterminedconcentration for a reactive component, providing a metallic powder,providing a controlled atmosphere, and additively manufacturing themetallic article from the metallic powder under the controlledatmosphere. The metallic powder includes a metallic element or metallicalloy and the reactive component at the predetermined concentration. Thereactive component reacts with the metallic powder in a weld pool formedduring the additive manufacturing to form a dispersion of nano-particlesin the weld pool. The nano-particles are dispersed throughout themetallic article in a substantially uniform manner.

Additional features and aspects of the present invention will becomeapparent to those of ordinary skill in the art upon reading andunderstanding the following detailed description of the exemplaryembodiments. As will be appreciated by the skilled artisan, furtherembodiments of the invention are possible without departing from thescope and spirit of the invention. Accordingly, the descriptions are tobe regarded as illustrative and not restrictive in nature.

DETAILED DESCRIPTION OF THE INVENTION

Exemplary embodiments of the present invention are described below.Although the following detailed description contains many specifics forpurposes of illustration, a person of ordinary skill in the art willappreciate that many variations and alterations to the following detailsare within the scope of the invention. Accordingly, the followingembodiments of the invention are set forth without any loss ofgenerality to, and without imposing limitations upon, the claimedinvention.

The present invention relates generally to additive manufacturingprocesses, and more specifically to systems and methods for formingnano-particles, such as, for example, nano-oxide particles, in certaintypes of metal alloys, such as, for example, copper-based alloys createdby additive manufacturing.

Additive manufacturing, as used herein, refers to any three-dimensional(3D) printing process by which a metallic article is formed using adirected energy source to additively melt and fuse a metallic powderlayer-by-layer to build up the article. Additive manufacturing processesinclude, but are not limited to, selective laser melting (SLM),selective laser sintering (SLS), direct metal laser sintering (DMLS),direct metal laser melting (DMLM), and electron beam melting (EBM).

Selective laser melting, as used herein, refers to any additivemanufacturing process using a laser beam as the directed energy source.Selective laser melting processes include, but are not limited to, SLS,DMLS, and DMLM.

A nano-particle, as used herein, refers to any dispersoid particleformed in a melt pool during an additive manufacturing process byreaction between a reactive component and a metal. In some embodiments,the nano-particle has a size in the range of about 1 nm to about 100 nm,alternatively in the range of about 1 nm to about 200 nm, alternativelyin the range of about 1 nm to about 1000 nm, or any value, range, orsub-range therebetween, depending on the conditions under which thenano-particle is formed.

A non-reactive gas, as used herein, refers to a gas that does not reactwith the melt pool formed by the metallic powder of a metallic elementor alloy during an additive manufacturing process. As certain gases mayreact with one metallic element or alloy but not another, whether aparticular gas is non-reactive may be dependent on the specific metallicelement or alloy being used to additively manufacture an article.

A reactive component, as used herein, refers to a compound or elementthat reacts with the melt pool formed by the metallic powder of ametallic element or alloy during an additive manufacturing process. Thereactive component may be provided in the atmosphere of the additivemanufacturing device or in the powder from which the article isadditively manufactured.

One aspect of this invention involves modifying or otherwisemanipulating the composition of the metallic powder and/or thecomposition of the gas or gases contained in the atmosphere inside theprinting chamber used in an additive manufacturing system (referred toas “headspace modulation”) to form metal compounds that confer desiredcharacteristics such as nano-particles. In some embodiments, theatmosphere includes primarily a non-reactive gas but also apredetermined level of a reactive component. In some embodiments, theatmosphere is a reduced pressure atmosphere or vacuum with apredetermined concentration of a reactive component. In someembodiments, the powder includes a predetermined concentration of areactive component. In some embodiments, the predetermined concentrationof the reactive component is selected to produce an article havingspecific mechanical properties or a specific concentration ofnano-particles.

The metallic powder may have any composition that reacts with a reactivecomponent. In some embodiments, the metallic powder is a pure metallicelement. In some embodiments, the metallic powder is a metallic alloy.In some embodiments, the metallic powder is pure copper, a copper-basedalloy, a copper-tin alloy, or a copper-nickel-silicon alloy. In someembodiments, the metallic powder is an iron-based alloy or a steelalloy. In some embodiments, the metallic powder is not a steel alloy. Insome embodiments, the metallic powder is not an iron-based alloy.

In some embodiments, the reactive component is oxygen. The unique effectfrom the additive manufacturing process, however, may not be limited tothe use of oxygen as the reactive component to form of nano-oxideparticles, but may be extended to other metal compounds that may beformed in the presence of suitable gases within the chamber gases, suchas nitrides (through the presence of small quantities of nitrogen in avacuum or an argon atmosphere) or silicides/carbides (through thepresence of volatile silicon or carbon compounds in a vacuum or in anitrogen or an argon atmosphere). In some embodiments, the reactivecomponent is not oxygen but instead is a reactive component other thanoxygen. Changing the relative concentrations and/or the composition ofthe purge gases surrounding the printing parts would presumably controlthe type and quantity of other metal compounds within the printed parts,thereby yielding desired improvements in mechanical properties.

The melt pool formed during the additive manufacturing process isunderstood to be very dynamic, with significant flow occurring withinthe melt pool leading to significant mixing, stirring, and entraining.This movement permits an increased level of incorporation of a reactivecomponent from the atmosphere into the melt pool and also an increasedrate of reaction between the reactive component and the metallic elementor alloy to form the nano-particles. In some embodiments, the amount offormed dispersoids from the additive manufacturing and precipitates fromthe precipitation hardening is greater than what is possible bysolutionizing and precipitation hardening for the same metallicelement/alloy. The rapid cooling which occurs upon removal of thedirected heat source essentially then leads to freezing of thenano-particles in place and a relatively homogeneous distribution of thenano-particles in the additively-manufactured article.

The rapid cooling of the melt pool in the additive manufacturing processprovides homogeneity in the formed article similar to a firstsolutionizing step of a conventional precipitation hardening process ofa wrought material. With regard to certain copper/nickel/silicon(CuNiSi) alloys, the micro-precipitation of a Ni—Si compound improvesthe mechanical properties, and the additional formation of thenano-oxide particles further enhances the mechanical properties. Theseenhanced mechanical properties are maintained over an extended exposureof the alloy to elevated temperatures that would normally have resultedin a decrease in such mechanical properties for a wrought form of thesame alloy. Accordingly, precipitation hardening from nano-particlesformed during an additive manufacturing process may be utilized tocreate Cu-based structures that maintain their mechanical properties forlonger time periods at elevated temperatures. Such materials arebeneficial for electrical connector applications in harsh environments,where extreme temperatures typically degrade the performance ofconventional copper-based contacts, or for other applications thatrequire copper-based parts that maintain their structural integrity andelectrical characteristics for prolonged periods of time.

As previously discussed, the precipitation of fine inclusions(precipitation hardening) is a mechanism for modification of themechanical properties of metal/metal alloys. In the processing ofmetals, the approach typically used to create a particular material isto form a melt with a desired composition, solidify that material, andthen apply a variety of post-solidification heat treatments to thematerial. However, the compositions attainable (and consequently thetypes of precipitated inclusions that are possible) are generallylimited to those that form melts.

The precipitation hardening process confers greater hardness to thearticle subjected to the process. Heat aging processes increase themechanical properties of metals up to a maximum before experiencing aloss of the enhanced mechanical properties as the exposure of thematerials to elevated temperatures continues. The systems and methods ofthe present invention provide additively-manufactured metals and alloysthat may be precipitation hardened in a simpler and more efficientmanner than the wrought versions of these alloys and that may retain thebeneficial aspects of precipitation hardening for a longer period oftime or at higher temperatures due to the presence of nano-particles inthe materials, either as additively-manufactured or after precipitationhardening.

Copper-based alloys made by an SLM process have been observed to includedispersoids in the form of nano-particles, more specifically nano-oxideparticles, and such alloys possess certain advantageous characteristicscompared to the wrought forms of the same or similar alloys. Inaddition, the very rapid cooling of materials formed by additivemanufacturing simplifies the process of precipitation hardening of suchmaterials by making the solution treatment step (i.e., the first step inthe process) unnecessary. Accordingly, the present invention includesmethods for modifying or manipulating the additive manufacturing processto affect the formation of advantageous dispersoids.

In some embodiments, the enhanced mechanical properties of the articleinclude a tensile strength, more specifically an ultimate tensilestrength, that is better maintained at an elevated temperature over anextended period of time. This enhanced mechanical property provides atleast two potential advantages. First, it makes the article easier toprocess than a wrought article, because the length of time for theprecipitation hardening is less critical for achieving a predeterminetensile strength or ultimate tensile strength. Second, it permits use ofthe article for a longer period of time without risk of failure in ahigh-temperature application compare to a wrought article. In someembodiments, the enhanced mechanical properties include an enhancedtensile strength, more specifically an enhanced ultimate tensilestrength. In some embodiments, the enhanced mechanical propertiesinclude an enhanced tensile strength, more specifically an enhancedultimate tensile strength, that is better maintained at an elevatedtemperature over an extended period of time.

EXAMPLES

An article was formed by an additive manufacturing process, morespecifically an SLM process of DMLS/DMLM, from a powder of a copper-tin(Cu—Sn) Cu-4% Sn alloy (95.5-96.5 wt % copper and 3.5-4.5 wt % tin;bronze). In analyzing the structural characteristics of the article, thenano-oxide particles of the additively-manufactured alloy were observedto be very stable at an annealing temperature of 600° C. (1100° F.).Dispersed nano-oxide particles were identified within the bulk of theSLM-formed Cu-4% Sn alloy.

With regard to the Cu-4% Sn alloy, the observed nano-oxide particles areassumed to have been continuously created during the SLM process andwere observed to be dispersed throughout the bulk interior of the copperalloy. The nano-oxide particles may be formed either as a result of themolten metal pool scavenging residual oxygen present in the otherwiseinert (nitrogen) atmosphere within the deposition chamber or from tracesof oxides in the metallic powders used in the SLM process.

An article was formed by an additive manufacturing process, morespecifically an optimized SLM process of DMLS/DMLM, from a powder of acommercial Corson alloy based on CuNiSi, referred to as 70250, having acomposition of 2.2-4.2 wt % Ni, 0.25-1.2 wt % Si, 0.05-0.30 wt %magnesium (Mg), up to 0.20 wt % iron (Fe), up to 1.0 wt % zinc (Zn), upto 0.1 wt % manganese (Mn), up to 0.05 wt % lead (Pb), and a balance ofCu. The article was then subjected to a precipitation hardening processto maximize its physical and mechanical properties. The nano-particlesin the article after the precipitation hardening were observed to besubstantially spherical with an average diameter of about 33 nm and tobe present at a concentration of about 0.25 vol % of the article.Similar to other copper-based alloys, Corson alloys (i.e., alloys thatderive their enhanced mechanical properties from a precipitation processcarried out at elevated temperatures) demonstrate decreased mechanicalproperties under longer aging times at elevated temperatures as a resultof a strengthening precipitate coarsening process. In a separableelectrical contact that depends on a spring force to maintain goodconductivity, the loss of mechanical properties may result in adecreased normal force on the separable interface and a degradation ofthe electrical performance across the separable interface.

The precipitation hardening process followed the additive manufacturingof the CuNiSi-based article without a solutionizing step between theadditive manufacturing and the precipitation hardening. Theprecipitation hardening process was thus simplified because an initialsolutionizing step was not required.

In this embodiment, the Corson alloy based on CuNiSi (70250) alloypowder was created using an optimized SLM process for printing highdensity parts and components. This process, which optimized laser power,laser travel speed, beam focus, spacing between laser lines, beamoffsets, and width of laser raster scan, was developed to: (i) obtain arelatively smooth and defect free finish on external surfaces; (ii)attain interior sections with a density of 98%-100% relative to thereported density of the wrought form of the alloy; and (iii) createstrong and continuous support structures that sufficiently bond to steelbuild plates as well as to the initial layers or printed parts. Thelaser parameters for a commercially available bronze alloy were found tobe inadequate to process the CuNiSi alloy. Accordingly, the laser energy(power/laser scan speed) was increased and a more focused beam settingwas applied.

Another aspect of this invention involves the formation ofnano-particles, specifically nano-oxide particles, inadditively-manufactured CuNiSi alloys. Precipitation-hardened CuNiSimaterials normally experience a decrease in mechanical properties whenexposed to elevated temperatures for extended times. As discussed above,nano-oxide particles were identified within the bulk of both a Cu-4% Snalloy (bronze) and a CuNiSi alloy, and are believed to be responsiblefor an observed reduced rate of microstructural grain coarsening at 600°C. (1100° F.). These nano-oxide particles are presumably created duringthe additive manufacturing process, which disperses the particlesthroughout the bulk of the interior of the material. The nano-oxideparticles may originate from trace oxide on the surface of the metallicpowder used in the DMLS printing process or may be created during theadditive manufacturing process by the molten metal scavenging traceoxygen from the otherwise inert atmosphere within the internalenvironment of an additive manufacturing machine/system. In either case,the oxide particles are dispersed throughout the bulk interior of theprinted material. Laboratory observations suggest that a similarphenomenon occurs in a CuNiSi alloy and may be responsible for enhancingthe mechanical properties during an over-aging condition when fabricatedusing DMLS. The unique effect from the additive manufacturing processmay not be limited to Cu/Sn alloys, but are expected to extend to othercopper-based alloys, and even other metal/metal alloy systems. Theunique effect(s) of the observed nano-particles are not expected to belimited to oxide containing particles, but to extend to other reactivecomplexes with metals.

Although only certain specific reactive components and metallic elementsand alloys are described herein, the methods described herein may beapplied to any pair of a reactive component and a metallic element oralloy to produce nano-particles in an additively manufactured article.As such, the metallic element or alloy may be any composition capable ofreacting with a reactive component when in a melted state to form anano-particle dispersoid in a melt pool of the metallic element oralloy. In some embodiments, the additively-manufactured article has acomposition that is not capable of being precipitation hardened afterbeing formed. In some embodiments, the additively-manufactured articleis subsequently subjected to a precipitation hardening process, asdescribed herein.

While the present invention has been illustrated by the description ofexemplary embodiments thereof, and while the embodiments have beendescribed in certain detail, it is not the intention to restrict or inany way limit the scope of the appended claims to such detail.Additional advantages and modifications will readily appear to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to any of the specific details, representative devices andmethods, and/or illustrative examples shown and described. Accordingly,departures may be made from such details without departing from thespirit or scope of the general inventive concept.

What is claimed is:
 1. A method of producing a metallic article, themethod comprising: providing a metallic powder, wherein the metallicpowder comprises a metallic element or metallic alloy; selecting apredetermined concentration for a reactive component; providing acontrolled atmosphere comprising the reactive component at thepredetermined concentration; and additively manufacturing the metallicarticle from the metallic powder under the controlled atmosphere suchthat the reactive component reacts with the metallic powder in a weldpool formed during the additive manufacturing to form a dispersion ofnano-particles in the weld pool; wherein the nano-particles aredispersed throughout the metallic article in a substantially uniformmanner.
 2. The method of claim 1, wherein the metallic powder is copperor a copper-based alloy.
 3. The method of claim 2, wherein thecopper-based alloy is a copper-nickel-silicon alloy or a copper-tinalloy.
 4. The method of claim 1, wherein the controlled atmospherefurther comprises an inert gas.
 5. The method of claim 4, wherein theinert gas is selected from the group consisting of argon, nitrogen, anda combination thereof.
 6. The method of claim 1, wherein the controlledatmosphere is a vacuum.
 7. The method of claim 1, wherein the reactivecomponent comprises an element selected from the group consisting ofoxygen, nitrogen, silicon, carbon, and a combination thereof.
 8. Themethod of claim 1, wherein the nano-particles are nano-oxide particles.9. The method of claim 1 further comprising subjecting the metallicarticle to a single-step precipitation hardening process, withoutsolutionizing the metallic article between the additive manufacturingand the precipitation hardening, to enhance at least one mechanicalproperty of the metallic article.
 10. The method of claim 1, wherein theadditive manufacturing comprises selective laser melting or electronbeam melting.
 11. A metallic article formed by the method of claim 1.12. A method of producing a metallic article, the method comprising:selecting a predetermined concentration for a reactive component;providing a metallic powder, wherein the metallic powder comprises ametallic element or metallic alloy and the reactive component at thepredetermined concentration; providing a controlled atmosphere; andadditively manufacturing the metallic article from the metallic powderunder the controlled atmosphere such that the reactive component reactswith the metallic powder in a weld pool formed during the additivemanufacturing to form a dispersion of nano-particles in the weld pool;wherein the nano-particles are dispersed throughout the metallic articlein a substantially uniform manner.
 13. The method of claim 12, whereinthe metallic element or metallic alloy is copper, a copper-based alloy,a copper-nickel-silicon alloy or a copper-tin alloy.
 14. The method ofclaim 12, wherein the controlled atmosphere is an inert gas atmosphere.15. The method of claim 14, wherein the inert gas is selected from thegroup consisting of argon, nitrogen, and a combination thereof.
 16. Themethod of claim 12, wherein the controlled atmosphere is a vacuum. 17.The method of claim 12, wherein the reactive component comprises anelement selected from the group consisting of oxygen, nitrogen, silicon,carbon, and a combination thereof.
 18. The method of claim 12, whereinthe nano-particles are nano-oxide particles.
 19. The method of claim 12further comprising subjecting the metallic article to a single-stepprecipitation hardening process, without solutionizing the metallicarticle between the additive manufacturing and the precipitationhardening, to enhance at least one mechanical property of the metallicarticle.
 20. The method of claim 12, wherein the additive manufacturingcomprises selective laser melting or electron beam melting.